Fix parser error pointing to wrong line when last line has no trailing newline (#1041)

When parsing a key without '=' at EOF (e.g., "a = 1\nb = 2\nc"), the
error highlight was an empty slice, causing subsliceOffset to return 0
and the error to point at line 1 instead of line 3. Pass the consumed
key bytes as the highlight instead of the empty remainder.

Fixes #1032

https://claude.ai/code/session_01UWv8pyc8P1ktAPfHpveixj

Co-authored-by: Claude <noreply@anthropic.com>

unstable/parser.go

@@ -345,7 +345,7 @@ func (p *Parser) parseKeyval(b []byte) (reference, []byte, error) {
 	b = p.parseWhitespace(b)
 
 	if len(b) == 0 {
-		return invalidReference, nil, NewParserError(b, "expected = after a key, but the document ends there")
+		return invalidReference, nil, NewParserError(startB[:len(startB)-len(b)], "expected = after a key, but the document ends there")
 	}
 
 	b, err = expect('=', b)